In this type of module the wheel is generally coupled to the motor by a rigid kinematic chain which does not permit any play in the transmission or of the rotational couple apart from the customary play resulting from the tolerances in machine finished parts. In other words, the irregularities of the road or small obstacles (stones, unexpected bumps in the road surface) or the jolts caused by braking or accelerating are immediately transmitted from one end to the other of the kinematic chain (from the wheel to the motor or vice versa). The result is premature fatigue of the parts in which there is play and consequently more or less rapid wear thereof.
To avoid these difficulties it is useful to be able to include in the kinematic chain a predetermined and temporary difference between the input motor couple and the useful output couple in such a manner that the major part of the effects of these sudden dynamic differences is absorbed.